Computed tomography is an imaging procedure that has been widely used in the medical field. In a procedure for computed tomography, an x-ray source and a detector apparatus are positioned on opposite sides of a portion of a patient under examination. The x-ray source generates and directs a x-ray beam towards the patient, while the detector apparatus measures the x-ray absorption at a plurality of transmission paths defined by the x-ray beam during the process. The detector apparatus produces a voltage proportional to the intensity of incident x-rays, and the voltage is read and digitized for subsequent processing in a computer. By taking a plurality of readings from multiple angles around the patient, relatively massive amounts of data are thus accumulated. The accumulated data are then analyzed and processed for reconstruction of a matrix (visual or otherwise), which constitutes a depiction of a density function of a volume of the bodily region being examined. By considering one or more sections in the volume, a skilled diagnostician can often diagnose various bodily ailments such as tumors, blood clots, etc.
Computed tomography has found its principal application to examination of bodily structures or the like which are in a relatively stationary condition. However, currently available computed tomographic apparatus may not be able to generate tomographic images with sufficient quality or accuracy due to physiological movement of a patient. For example, breathing has been known to cause degradation of quality in CT images. Existing CT imaging systems may use a camera to determine patient position. In such CT imaging system, a marker block having a plurality of markers is placed on a patient's chest. The camera is then used to sense the markers on the marker block. By determining the positions of the markers, a position and/or orientation of the marker block can be determined. The positional data is then considered in the construction of CT images so that patient's movement is addressed.
Existing treatment systems have also used a camera to determine a position of a patient during a treatment procedure. For example, in a radiation system, a marker block having a plurality of markers may be placed on a patient's chest. The camera is then used to sense the markers, thereby determining breathing phases and/or breathing amplitudes of the patient. The phases or amplitudes may then be used to gate a delivery of a treatment radiation beam towards a target region.
In existing systems, before the single camera is used, the camera is calibrated to determine the position of the camera. In a single camera calibration procedure, a target (e.g., a marker) position is known, and the camera is used to obtain an image of the target. The position of the camera is then estimated that will result in the image obtained by the camera. Calibration of a single camera system has been described in U.S. Pat. No. 6,973,202. Once the camera position is known, it can then be used in a tracking scheme. During a tracking scheme, the image of a target is obtained using the camera, and the position of the target is estimated that will result in the image seen by the camera. As such, in the single camera case, the scheme for tracking and the scheme for calibration are conjugate. As a result, when tracking a target from a previously calibrated camera, any systematic errors introduced by the calibration method will tend to cancel out the errors introduced by the tracking method.
Since single camera tracking tends to be insensitive for motion along the camera axis, it may be desirable to use two cameras for tracking object position using triangulation. When using dual camera system in a tracking scheme, the camera positions are known, and the two cameras are used to obtain images of a target. The position of the target is then determined using triangulation. In dual camera systems, the cameras may be calibrated by performing two single camera calibrations. However, such calibration technique is not conjugate to the tracking method, which uses triangulation. As a result, a systematic error introduced during the calibration scheme may not be offset by error introduced in the tracking scheme.
In addition, calibration methods that are used in single camera systems may not apply equally well in dual-camera systems. In particular, since in dual-camera calibration setups, the two cameras are looking simultaneously at the same calibration target, there is information about the setup that is not being utilized by an algorithm that considers the cameras to be calibrated separately. As such, it is desirable to use an algorithm that takes advantage of the intrinsic properties of a dual-camera setup for calibrating one or both of the cameras.